Cement slurries are used to support the intermediate casting pipe in geothermal wells, and to protect the casing from corrosive fluids and gases. The use of normal density cementing slurries (14 to 15.8 lb/gal) or 1.7 to 1.9 g/cc) in geothermal wells frequently results in lost circulation problems, particularly in cement well regions which pass through weak unconsolidated zones. Specifically, these weak zones fail as the result of high hydrostatic pressures required to pump the highly dense cement slurries. One object of the present invention is to provide low density cement slurries, and the process for producing these slurries, which remain stable under high temperature, high pressure conditions.
The foamed cement slurries contemplated by the present invention must be capable of stably withstanding the conditions found in high-enthalpy geothermal wells. These conditions include hydrothermal temperatures of up to 350.degree. C. and hydrostatic pressures up to 2,000 psi.
It is therefore an object of this invention to provide compositions and processes which result in stable foamed cement whose mechanical, physical, and microstructural characteristics are such that these cements can withstand the conditions in high-enthalpy geothermal wells.
It is also an object of this invention to provide processes and compositions for cements capable of stably withstanding 350.degree. C. and pressures up to 2,000 psi.
It is also an object of this invention to provide lightweight cements with sufficient compressive strength and water permeability to stably withstand high-enthalpy geothermal well conditions.
Another object of the present invention is the microscopy observations of the surface microstructure of the foam cements of the present invention show that these stabilized foam cements contain a foam cements show that the stabilized foam cement contained a uniform distribution of the fine bubbles in the foam size range of 5 .mu.m to 20 .mu.m (unstabilized cement, in comparison, displays a range of 30 .mu.m to 200 .mu.m). The high quality foam cements of the present invention, therefore, contain uniformly distributed fine foam bubbles, thereby improving the mechanical properties and water permeability of the set cement.
Another object of the present invention is the production of reinforced foam cements with high strength. The highest strength achieved (2,310 psi) is found in a cement composition with graphite fiber embedded into the cement matrix. The strength value corresponds to an improvement value of about 1.3 times compared to the control, attributable to graphite's inert reaction to strong alkaline media at high temperatures.
Other objects and characteristics of the cements of the present invention will become evident from the following description.